Extreme ultraviolet mask with improved absorber

ABSTRACT

The present invention discloses an EUV mask having an improved absorber layer with a certain thickness that is formed from a metal and a nonmetal in which the ratio of the metal to the nonmetal changes through the thickness of the improved absorber layer and a method of forming such an EUV mask.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor integratedcircuit manufacturing, and more specifically, to a mask and a method offabricating a mask used in extreme ultraviolet (EUV) lithography.

2. Discussion of Related Art

Continual improvements in optical lithography have allowed the shrinkageof semiconductor integrated circuits (IC) to produce devices with higherdensity and better performance. Decreasing the wavelength used forexposure improves resolution and mitigates the effects of diffraction.Deep ultraviolet (DUV) light with a wavelength of 248 or 193 nanometers(nm) is widely used for exposure through a transmissive mask fabricatedfrom a quartz substrate. DUV light with a wavelength of 157 or 126 nmmay be used for exposure through a transmissive mask made from CalciumFluoride. However, at around the 70-nm node, Next Generation Lithography(NGL) will be needed.

NGL includes EUV lithography, electron projection lithography (EPL), andproximity x-ray lithography (PXL). PXL has been characterized the mostextensively, but this technology is constrained by the requirement for asynchroton source and the difficulty of scaling down the 1X-masks. EPLis suitable for application specific integrated circuits (ASIC), butthroughput is significantly degraded whenever complementary structuresare patterned since two passes are required with a stencil mask. EUVlithography is best suited for manufacturing of memory chips andmicroprocessors since the high costs of an EUV mask and an EUV sourcecan be spread over a higher volume of product.

EUV lithography is based on exposure with the portion of theelectromagnetic spectrum having a wavelength of 10-15 nanometers. An EUVstep-and-scan tool may have a 4-mirror, 4×-reduction projection systemwith a 0.10 Numerical Aperture (NA). Exposure is accomplished bystepping fields over a wafer and scanning an arc-shaped region of theEUV mask across each field. A critical dimension (CD) of 50-70 nm may beachieved with a depth of focus (DOF) of about 1 micrometer (um).Alternatively, an EUV step-and-scan tool may have a 6-mirror,4×-reduction projection system with a 0.25 NA to print a smaller CD of20-30 nm, at the expense of a reduced DOF. Other tool designs with a 5×-or a 6×-reduction projection system may also be used for EUVlithography.

A mask for DUV lithography is transmissive. Thus, the desired pattern ona DUV mask is defined by selectively removing an opaque layer, such asChrome, to uncover portions of an underlying transparent substrate, suchas quartz. However, virtually all condensed materials absorb at the EUVwavelength so a mask for EUV lithography is reflective. Consequently,the desired pattern on an EUV mask is defined by selectively removing anabsorber layer to uncover portions of an underlying mirror coated on asubstrate.

Thus, what is needed is an EUV mask with an improved absorber layer anda method of fabricating such an EUV mask.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-(d) are illustrations of a cross-sectional view of an EUVmask blank with an improved absorber layer formed according to thepresent invention.

FIGS. 2(a)-(d) are illustrations of a cross-sectional view of an EUVmask with an improved absorber layer formed according to the presentinvention.

FIG. 3 is an illustration of a cross-sectional view of an EUV mask withan improved absorber layer according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following description, numerous details, such as specificmaterials, dimensions, and processes, are set forth in order to providea thorough understanding of the present invention. However, one skilledin the art will realize that the invention may be practiced withoutthese particular details. In other instances, well-known semiconductorequipment and processes have not been described in particular detail soas to avoid obscuring the present invention.

Nearly all materials absorb Extreme Ultraviolet (EUV) light so a maskfor EUV lithography is reflective. A pattern on an EUV mask is definedby selectively removing portions of an absorber layer to uncover anunderlying mirror. The absorber layer controls critical dimension (CD),defect level, and registration. Thus, the absorber layer on an EUV maskhas a direct impact on the quality of the mask. The present invention isan EUV mask with an improved absorber layer and a method of fabricatingsuch an EUV mask.

Various embodiments of a process for fabricating an EUV mask accordingto the present invention will be described next. First, as shown in FIG.1(a), a substrate 1100 with a low defect level and a smooth surface isused as the starting material for an EUV mask of the present invention.It is desired to form the substrate 1100 out of a glass or glass-ceramicmaterial that has a low coefficient of thermal expansion (CTE). Incertain cases, the substrate 1100 may be formed from Silicon despite itslarge CTE as long as heat can be removed uniformly and effectivelyduring exposure.

Second, as shown in FIG. 1(b), a multilayer (ML) mirror 1200 is formedover the substrate 1100. The ML mirror 1200 has about 20-80 pairs ofalternating layers of a high index of refraction material 1210 and a lowindex of refraction material 1220. A high index of refraction material1210 includes elements with high atomic number which tend to scatter EUVlight. A low index of refraction material 1220 includes elements withlow atomic number which tend to transmit EUV light.

The choice of materials in the ML mirror 1200 depends on theillumination wavelength, lambda. To a first approximation, each layerhas a thickness of about one quarter of lambda. More specifically, thethickness of the individual layers depends on the illuminationwavelength, lambda, and the incidence angle of the illumination light.For EUV, the wavelength is about 13.4 nm and the incidence angle isabout 5 degrees. The thicknesses of the alternating layers are tuned tomaximize the constructive interference of the EUV light reflected ateach interface and to minimize the overall absorption of the EUV light.The ML mirror 1200 can achieve about 60-75% reflectivity at the peakillumination wavelength.

In one embodiment, the ML mirror 1200 has 40 pairs of alternating layersof a high index of refraction material 1210 and a low index ofrefraction material 1220. For example, the high index of refractionmaterial 1210 may be formed from about 2.8 nm thick Molybdenum (Mo)while the low index of refraction material 1220 may be formed from about4.1 nm thick Silicon (Si). As needed, a capping layer 1230, such asabout 11.0 nm thick Silicon (Si), may be formed at the top of the MLmirror 1200 to prevent oxidation of Molybdenum by exposure to theenvironment.

The ML mirror 1200 is formed over the substrate 1100 by using ion beamdeposition (IBD) or DC magnetron sputtering. The thickness uniformityshould be better than 0.8% across the substrate 1100. IBD results inless perturbation and fewer defects in the upper surface of the MLmirror 1200 because the deposition conditions can usually be optimizedto smooth over any defect on the substrate 1100. DC magnetron sputteringis more conformal, thus producing better thickness uniformity, but anydefect on the substrate 1100 will tend to propagate up through thealternating layers to the upper surface of the ML mirror 1200.

Third, as shown in FIG. 1(c), a buffer layer 1300 is formed over the MLmirror 1200. The buffer layer 1300 may have a thickness of about 20-105nm. The buffer layer 1300 may be formed from Silicon Dioxide (SiO₂),such as a low temperature oxide (LTO). A low process temperature,typically less than about 150 C., is desirable to prevent interdiffusionof the alternating layers in the underlying ML mirror 1200. Othermaterials, such as Silicon Oxynitride (SiOxNy) or Carbon (C) may also beused for the buffer layer 1300. The buffer layer 1300 may be depositedby RF magnetron sputtering.

Fourth, as shown in FIG. 1(d), an improved absorber layer 1400 is formedover the buffer layer 1300. The improved absorber layer 1400 may beformed from one type of material or from a stack of different materials.Variation in the layer, if present, may be continuous or may be discretewith distinct interfaces. In one embodiment, the improved absorber layer1400 is formed from Tantalum Nitride (TaN) in which the stoichiometry ischanged through the thickness of the film.

The improved absorber layer 1400 has a total thickness of about 45-215nm. In most cases, the improved absorber layer 1400 may be deposited byRF sputtering. Depending on the material selected, the improved absorberlayer 1400 may be deposited by DC sputtering. In some cases, theimproved absorber layer 1400 may be deposited by ion beam deposition(IBD) or atomic layer chemical vapor deposition (ALC).

The improved absorber layer 1400 may be formed entirely or partially outof one or more metals and their borides, carbides, nitrides, oxides,phosphides, or sulfides. Examples of suitable metals include Tantalum(Ta), Tungsten (W), Hafnium (Hf), and Niobium (Nb).

The improved absorber layer 1400 may also be formed from amorphousalloys of metals. Examples include Tantalum Silicon (TaSi) and TantalumGermanium (TaGe).

As shown in FIG. 1(d), the combination of an improved absorber layer1400, buffer layer 1300, ML mirror 1200, and substrate 1100 results inan EUV mask blank 1700. The EUV mask blank 1700 shown in FIG. 1(d) canbe further processed to produce an EUV mask 1800 as shown in FIG. 2(d).

First, as shown in FIG. 2(a), an EUV mask blank 1700 is covered with aradiation-sensitive layer, such as photoresist 1600, that is coated,exposed, and developed with a desired pattern. The photoresist 1600 hasa thickness of about 160-640 nm. As desired, a chemically-amplifiedresist (CAR) may be used. Depending on the photoresist 1600 used,exposure is performed on an electron beam (e-beam) writer or a laserwriter.

After post-develop measurement of the critical dimension (CD) of thefeatures in the pattern in the photoresist 1600, the pattern istransferred into the improved absorber layer 1400 as shown in FIG. 2(b).Reactive ion etch may be used. For example, an improved absorber layer1400 may be dry etched with a gas which contains Chlorine, such as Cl₂or BCl₃, or with a gas which contains Fluorine, such as NF₃. Argon (Ar)may be used as a carrier gas. In some cases, Oxygen (O₂) may also beincluded. The etch rate and the etch selectivity depend on power,pressure, and substrate temperature.

The buffer layer 1300 serves as an etch stop layer to help achieve agood etch profile in the overlying improved absorber layer 1400. Thebuffer layer 1300 protects the underlying ML mirror 1200 from damageduring etch of the improved absorber layer 1400.

Reactive ion etch may not be feasible for patterning the improvedabsorber layer 1400 if the byproducts of etching are not volatile andthus cannot be removed from the chamber by pumping. Then the improvedabsorber layer 1400 may be patterned with a sputtering process in anArgon plasma. If desired, a dual frequency high-density plasma may beused. Alternatively, if selectivity to the masking material is adequate,ion milling may be used to pattern the improved absorber layer 1400.

Removal of the photoresist 1600 is followed by post-etch measurement ofthe CD of the features in the pattern in the improved absorber layer1400. The CD measurement may be done with a scanning electron microscope(SEM) or an optical metrology tool. Then defect inspection is done at awavelength typically between 150-500 nm. The defect inspection is basedon a comparison of the light signals in the patterned regions relativeto the non-patterned regions.

As shown in FIG. 2(b), defects may occur in the improved absorber layer1400 as a result of the pattern transfer from the photoresist 1600. Afirst type of defect is a clear defect 1710 while a second type ofdefect is an opaque defect 1720. In a clear defect 1710, the improvedabsorber layer 1400 should be present, but it is entirely or partiallymissing. In an opaque defect 1720, the improved absorber layer 1400should be removed, but it is entirely or partially present.

Repair of defects in the improved absorber layer 1400 is performed witha focused ion beam (FIB) tool as needed as shown in FIG. 2(c). A cleardefect 1710 is filled in with an opaque repair material 1730. An opaquedefect 1720 is removed, leaving a Gallium stain 1740 in the underlyingbuffer layer 1300. Thus, the buffer layer 1300 also protects theunderlying ML mirror 1200 from damage during repair of the improvedabsorber layer 1400.

The buffer layer 1300 increases light absorption over the ML mirror 1200when the EUV mask 1720 is used during exposure of photoresist on awafer. The resulting reduction in contrast can slightly degrade CDcontrol of the features printed in the photoresist on a wafer.Throughput is also decreased. As a result, it is desirable to remove thebuffer layer 1300 wherever it is not covered by the improved absorberlayer 1400 as shown in FIG. 2(d).

The buffer layer 1300 may be removed with a dry etch or a wet etch or acombination of the two processes. The dry etch or wet etch used toremove the buffer layer 1300 must not damage the overlying improvedabsorber layer 1400 or the underlying ML mirror 1200.

The buffer layer 1300 may be dry etched with a gas which containsFluorine, such as CF₄ or C₄F₈. Oxygen (O₂) and a carrier gas, such asArgon (Ar), may be included. A dry etch provides a steeper profile and asmaller undercut in the buffer layer 1300.

The buffer layer 1300 may also be wet etched, especially if it is verythin, since any undercut would then be very small. For example, a bufferlayer 1300 formed from Silicon Dioxide (SiO₂) may be etched with anaqueous solution of about 3-5% hydrofluoric (HF) acid. A wet etch cancompensate for larger variations in thickness of the buffer layer 1300.

The result of the process described above is an EUV mask 1800 having areflective region 1750 and a dark region 1760, as shown in FIG. 2(d).

Another embodiment of the present invention is an EUV mask 2700 as shownin FIG. 3. An EUV mask 2700 includes an improved absorber layer 2400, abuffer layer 2300, an ML mirror 2200, and a substrate 2100. The EUV mask2700 has a first region 2750 and a second region 2760. The first region2750 is reflective because the ML mirror 2200 is uncovered. The secondregion 2760 is darker due to the top layer 2500 and the improvedabsorber layer 2400.

First, the EUV mask 2700 of the present invention includes a substrate2100 with a low defect level and a smooth surface is used as thestarting material for an EUV mask of the present invention. Thesubstrate 2100 should have a low coefficient of thermal expansion (CTE).The substrate 2100 may be a low CTE glass or a low CTE glass-ceramic.However, in some cases, the substrate 2100 is Silicon. Although Siliconhas a large CTE that may result in undesirable displacement of printedimages, Silicon also has a high thermal conductivity and thus is aviable substrate as long as heat can be removed efficiently from themask during exposure.

Second, a multilayer (ML) mirror 2200 is disposed over the substrate2100. The ML mirror 2200 has about 20-80 pairs of alternating layers ofa high index of refraction material 2210 and a low index of refractionmaterial 2220. A high index of refraction material 2210 includeselements with high atomic number which tend to scatter EUV light. A lowindex of refraction material 2220 includes elements with low atomicnumber which tend to transmit EUV light.

The choice of materials in the ML mirror 2200 depends on theillumination wavelength, lambda. To a first approximation, each layerhas a thickness of about one quarter of lambda. More specifically, theoptimal thickness of the individual layers depends on the illuminationwavelength, lambda, and the incidence angle of the illumination light.For EUV, the wavelength is about 13.4 nm and the incidence angle isabout 5 degrees. The optimal thicknesses of the alternating layersmaximize the constructive interference of the EUV light reflected ateach interface and minimize the overall absorption of the EUV light. TheML mirror 2200 has about 60-75% reflectivity at the peak illuminationwavelength.

In one embodiment, the ML mirror 2200 has 40 pairs of alternating layersof a high index of refraction material 2210 and a low index ofrefraction material 2220. The high index of refraction material 2210 mayinclude Molybdenum (Mo) with a thickness of about 2.8 nm while the lowindex of refraction material 2220 may include Silicon (Si) with athickness of about 4.1 nm. As needed, a capping layer 2230, such asabout 11.0 nm thick Silicon (Si), may be included at the top of the MLmirror 2200 to prevent oxidation of Molybdenum by the environment.

Third, a buffer layer 2300 is disposed over the ML mirror 2200. Thebuffer layer 2300 may have a thickness of about 20-105 nm. The bufferlayer 2300 protects the underlying ML mirror 2200 from damage by actingas an etch stop layer during etch of the absorber layer 2400. The bufferlayer 2300 also protects the underlying ML mirror 2200 from damageduring repair of the improved absorber layer 2400.

The buffer layer 2300 may be Silicon Dioxide (SiO₂), such as lowtemperature oxide (LTO). Other materials, such as Silicon Oxynitride(SiOxNy) or Carbon (C) may also be used for the buffer layer 2300.

Fourth, an improved absorber layer 2400 is disposed over the bufferlayer 2300. The improved absorber layer 2400 may have a thickness ofabout 45-215 nm. The improved absorber layer 2400 must absorb EUV lightstrongly, remain physically and chemically stable during exposure to EUVlight, and be compatible with the entire mask fabrication process,including photoresist patterning, absorber etch, photoresist removal,FIB repair, and buffer layer removal. Control of the critical dimension(CD) must be maintained and line edge roughness (LER) must be minimized.

It is desirable for the improved absorber layer 2400 to be as thin aspracticable in order to minimize any shadowing effect during exposure.In one embodiment, the improved absorber layer 2400 is Tantalum Nitridewith a thickness of 50-100 nm. The EUV illumination beam typically hasan incidence angle of about 5 degrees so minimizing any shadowing duringexposure will reduce print bias. Similarly, minimizing any shadowingduring etch of the improved absorber layer 2400 will reduce etch bias.The etch bias for Tantalum Nitride can be reduced to less than 10 nm.Reducing print bias and etch bias will make it easier for the e-beamwriter to resolve the features to be patterned on the EUV mask 2700 andthus will permit the scaling to smaller design rules where the densefeatures are limited by the pitch. Tantalum Nitride should be extendibledown to the 30 nm design rule generation.

The CTE of the improved absorber layer 2400 should be fairly closelymatched with the CTE of the other materials in the EUV mask 2700. Ingeneral, a low coefficient of thermal expansion (CTE) is desirable forthe improved absorber layer 2400. Furthermore, a high thermalconductivity in the improved absorber layer 2400 is helpful inminimizing hot spots during exposure of the EUV mask 2700.

Reducing the thickness of the improved absorber layer 2400 is furtheradvantageous since the stress in the film will also be decreased.Mechanical stress in the film is undesirable since it may lead todistortion of the pattern on the EUV mask 2700 during e-beam writing ofthe EUV mask 2700. Excessive stress may also distort the pattern printedon the wafer during exposure of the EUV mask 2700.

Over a wide range of wavelength, lambda, the absorption coefficient ofan element with a density, rho, and an atomic number, Z, is proportionalto (rho)(Z)⁴(lambda)³. Elements from period 6 and group 4-11 of theperiodic table are potentially good candidates for the improved absorberlayer 2400. Examples of elements which have large rho and large Zinclude Hafnium (rho=13.30 g/cm³, Z=72), Tantalum (rho=16.60 g/cm³,Z=73), Tungsten (rho=19.35 g/cm³, Z=74), Rhenium (rho=20.53 g/cm³,Z=75), Osmium (rho=22.48 g/cm³, Z=76), Platinum (rho=21.45 g/cm³, Z=78),and Gold (rho=19.32 g/cm³, Z=79). These elements may all be depositedwith either DC sputtering or RF sputtering.

It is desirable for the improved absorber layer 2400 to have goodadhesion to the underlying buffer layer 2300 and, as needed, to anyoverlying opaque repair material. In most, but not all, cases, hardnessis also beneficial in contributing to the overall robustness of the EUVmask 2700. Tantalum forms good films. Tungsten films are hard andadherent, although the oxides are volatile. In certain cases, Rheniumfilms will self-evaporate. However, both Platinum films and Gold filmsare soft and have poor adhesion.

Certain alloys and ceramics are also suitable for the improved absorberlayer 2400. Examples of amorphous alloys include Tantalum Silicon andTantalum Germanium. Examples of ceramics include Tungsten Carbide(rho=17.15 g/cm³), Tantalum Nitride (rho=16.30 g/cm³), Tantalum Carbide(rho=13.90 g/cm³), Hafnium Carbide (rho=12.20 g/cm³), Tantalum Boride(rho=11.15 g/cm³), and Hafnium Boride (rho=10.50 g/cm³). These ceramicsmay be formed by DC sputtering or RF sputtering. Tungsten Boride(rho=10.77 g/cm³) may be formed by RF sputtering. Tantalum Nitride mayalso be formed by Reactive RF sputtering. Tantalum may be evaporated in10⁻³ Torr Nitrogen.

The improved absorber layer 2400 may include one type of material or maybe a stack of different materials. Variation in the layer, if desired,may be continuous or may be discrete with distinct interfaces. Forexample, in one embodiment, the improved absorber layer 2400 is a TaxNyfilm or Tantalum doped with Nitrogen in which x=1 and y<0.6. In anotherembodiment, the improved absorber layer 2400 is Tantalum Nitride (TaN)in which the stoichiometry changes through the thickness of the film.

Many embodiments and numerous details have been set forth above in orderto provide a thorough understanding of the present invention. Oneskilled in the art will appreciate that many of the features in oneembodiment are equally applicable to other embodiments. One skilled inthe art will also appreciate the ability to make various equivalentsubstitutions for those specific materials, processes, dimensions,concentrations, etc. described herein. It is to be understood that thedetailed description of the present invention should be taken asillustrative and not limiting, wherein the scope of the presentinvention should be determined by the claims that follow.

Thus, we have described an EUV mask with an improved absorber layer anda method of fabricating such an EUV mask.

We claim:
 1. A method comprising: providing a substrate; forming amultilayer mirror over said substrate, said multilayer mirror beingreflective at a wavelength; forming an improved absorber layer over saidmultilayer mirror, said improved absorber layer being absorbent at saidwavelength, said improved absorber layer having a thickness, saidimproved absorber layer comprising a first element doped with a secondelement, wherein a ratio of said second element to said first element isless than 0.6 to 1.0, wherein said ratio changes through said thickness;and removing said improved absorber layer in a first region whileleaving said improved absorber layer in a second region.
 2. The methodof claim 1 wherein said first element comprises Tantalum.
 3. The methodof claim 1 wherein said first element comprises Hafnium.
 4. The methodof claim 1 wherein said first element comprises Tungsten.
 5. The methodof claim 1 wherein said second element comprises Nitrogen.
 6. The methodof claim 1 wherein said second element comprises Carbon.
 7. The methodof claim 1 wherein said second element comprises Boron.